We report the best figure-of-merit achieved for an internal nuclear polarized hydrogen gas target and a Monte Carlo simulation of spin-exchange optical pumping. The dimensions of the apparatus were optimized using the simulation and the experimental results were in good agreement with the simulation. The best result achieved for this target was 50.5% polarization with 58.2% degree of dissociation of the sample beam exiting the storage cell at a hydrogen flow rate of $1.1\times 10^{18}$ atoms/s.
Recently, the production of ultrahigh-density (~10^{19}cm^{-3}) spin-polarized deuterium (SPD) atoms was demonstrated, from the photodissociation of deuterium iodide, but the upper density limit was not determined. Here, we present studies of spin-polarized hydrogen (SPH) densities up to 10^{20} cm^{-3}, by photodissociating 5 bar of hydrogen chloride with a focused 213 nm, 150 ps laser pulse. We extract the depolarization cross-section of hydrogen and chlorine atom collisions, which is the main depolarization mechanism at this high-density regime, to be σ_{HCl} = 7(2) x 10^{-17}cm^2. We discu…
In a recent publication [arXiv:2010.14579], we introduced a new type of atomic magnetometer, which relies on hydrohalide photo-dissociation to create high-density spin-polarized hydrogen. Here, we extend our previous work and present a detailed theoretical analysis of the magnetometer signal and its dependence on time. We also derive the sensitivity for a spin-projection noise limited magnetometer, which can be applied to an arbitrary magnetic field waveform.
We investigate the strong-field ionization of atomic hydrogen in a few-cycle elliptically polarized infrared pulse by solving the time-dependent Schrödinger equation. The dependence of the photoelectron momentum distribution on the pulse intensity, ellipticity, length, envelope, and carrier envelope phase is analyzed. In particular, we explain the variation of the electron offset angle with asymptotic electron energy through the combined action of the field and the Coulomb potential, and demonstrate that low-ellipticity pulses make it possible to access the electron release time.
Spectra of circularly polarized harmonics is calculated by numerically solving the Time-Dependent Schrödinger Equation for a 2D model of Ne atom using circularly polarized fundamental with counter-rotating second harmonic laser fields. We demonstrate strong asymmetry between left- and right- circularly polarized harmonics when a ground state with p-type symmetry is used. It arises due to the circular polarization of individual attosecond pulses in the generated pulse train. Reducing the length of the counter-rotating drivers and introducing a small time-shift between them allows to generate a …
We extend the semiclassical two-step model for strong-field ionization that describes quantum interference and accounts for the Coulomb potential beyond the semiclassical perturbation theory to the hydrogen molecule. In the simplest case of the molecule oriented along the polarization direction of a linearly polarized laser field, we predict significant deviations of the two-dimensional photoelectron momentum distributions and the energy spectra from the case of atomic hydrogen. Specifically, for the hydrogen molecule the electron energy spectrum falls off slower with increasing energy, and th…
We develop an adiabatic Floquet picture in the length gauge to describe the dynamics of a hydrogen atom in an intense laser field. In this picture, we discuss the roles played by frequency and intensity in terms of adiabatic potentials and the couplings between them, which gives a physical and intuitive picture for quantum systems exposed to a laser field. For simplicity, analyze hydrogen and give the adiabatic potential curves as well as some physical quantities that can be readily calculated for the ground state. Both linearly and circularly polarized laser fields are discussed.
We investigate the process of circularly polarized high harmonic generation in molecules using a bicircular laser field. In this context, we show that molecules offer a very robust framework for the production of circularly polarized harmonics, provided their symmetry is compatible with that of the laser field. Using a discrete time-dependent symmetry analysis, we show how all the features (harmonic order and polarization) of spectra can be explained and predicted. The symmetry analysis is generic and can easily be applied to other target and/or field configurations.
We consider electron-atom scattering in a circularly polarized laser field at sufficiently high electron energies, permitting to describe the scattering process by the first order Born approximation. Assuming the radiation field has sufficiently moderate intensities, the laser-dressing of the hydrogen target atom in its ground state will be treated in second order perturbation theory. Within this approximation scheme, it is shown that the nonlinear differential cross sections of free-free transitions do neither depend on the {\it dynamical phase} $ϕ$ of the radiative process nor on the {\it he…
We join the tribute to Professor N.B. Delone in this memorial issue by presenting the results of new calculations on the effects of ellipticity on double ionization by short and strong near-optical laser pulses.
We introduce a new method that allows one to obtain an analytical cross section for the laser-assisted electron-ion collision in a closed form. As an example we perform a calculation for the hydrogen laser-assisted recombination. The $S$-matrix element for the process is constructed from an exact electron Coulomb-Volkov wave function and an approximate laser modified hydrogen state. An explicit expression for the field-enhancement coefficient of the process is expressed in terms of the dimensionless parameter $κ= |{eε_{0}}/{qω_{0}}|^{2}$, where $e$ and $q$ are the electron charge and momentum …
We present analytical and numerical results of the relativistic calculation of the transition matrix element $S_{fi}$ and differential cross section for Mott scattering of initially polarized Dirac particles (electrons) in the presence of strong laser field with linear polarization. We use exact Dirac-Volkov wave functions to describe the dressed electrons and the collision process is treated in the first Born approximation. The influence of the laser field on the degree of polarization of the scattered electron is reported.
We propose a laser cooling technique in which atoms are selectively excited to a dressed metastable state whose light shift and decay rate are spatially correlated for Sisyphus cooling. The case of cooling magnetically trapped (anti)hydrogen with the 1S-2S-3P transitions using pulsed ultra violet and continuous-wave visible lasers is numerically simulated. We find a number of appealing features including rapid 3-dimensional cooling from ~1 K to recoil-limited, millikelvin temperatures, as well as suppressed spin-flip loss and manageable photoionization loss.
The full three-dimensional photoelectron momentum distributions of argon are measured in intense near-circularly polarized laser fields. We observed that the transverse momentum distribution of ejected electrons by 410-nm near-circularly polarized field is unexpectedly narrowed with increasing laser intensity, which is contrary to the conventional rules predicted by adiabatic theory. By analyzing the momentum-resolved angular momentum distribution measured experimentally and the corresponding trajectories of ejected electrons semiclassically, the narrowing can be attributed to a temporary trap…
New insights into our understanding of the semirelativistic excitation of atomic hydrogen by electronic impact have been made possible by combining the use of polarized electron beams and intense laser field. The paper reviews relativistic theoretical treatment in laser-assisted electron scattering with particular emphasis upon spin effects. Different spin configurations for inelastic electron-atom collisions is also discussed. The role of laser field in such collision is of major importance and reveals new information on the dynamics of the collision process. The examined modern theoretical i…
Molecular hydrogen ions are of metrological relevance due to the possibility of precise theoretical evaluation of their spectrum and of external-field-induced shifts. We report the results of the calculations of the rate of laser-induced electric quadrupole transitions between a large set of ro-vibrational states of ${\rm H_2^+}$. The hyperfine and Zeeman structure of the E2 transition spectrum and the effects of the laser polarization are treated in detail. We also present the nuclear spin-electron spin coupling constants, computed with a precision 10 times higher than previously.
We theoretically study multiphoton ionization through the triplet states of Mg by linearly polarized (LP) and circularly polarized (CP) fs laser pulses. After the construction of the atomic basis using the frozen-core Hartree-Fock potential (FCHFP) as well as the model potential (MP) approaches for both singlet and triplet series which show rather good agreements with the existing data in terms of state energies and dipole matrix elements, we solve time-dependent Schrödinger equations with $3s3p$ $^{3}P_{1}$ as an initial state, and calculate the total ionization yield and photoelectron energy…
We investigate the Rydberg states generation of Hydrogen atoms with intense laser pulses, by solving the time-dependent Schrödinger equation and by means of classical trajectory monte-carlo simulations. Both linearly polarized multi-cycle pulses and pairs of optical half cycle pulses are used. Comparisons between these methods show that both the Coulomb force and initial lateral momentum, which have effects on the $n$-distribution and $l$-distribution of the population of excited states, are important in the generation of Rydberg states.
We theoretically study frustrated double ionization (FDI) of atoms subjected to intense circularly polarized laser pulses using a three-dimensional classical model. We find a novel "knee" structure of FDI probability as a function of intensity, which is similar to the intensity dependence of nonsequential double ionization probability. The observation of FDI is more favourable when using targets with low ionization potentials and short driving laser wavelengths. This is attributed to the crucial role of recollision therein, which can be experimentally inferred from the photoelectron momentum d…
Based on the strong-field approximation, we obtain analytical expressions for the initial momentum at the tunnel exit and instantaneous ionization rate of tunneling ionization in elliptically polarized laser fields with arbitrary ellipticity. The tunneling electron reveals a nonzero offset of the initial momentum at the tunnel exit in the elliptically polarized laser field. We find that the transverse and longitudinal components of this momentum offset with respect to the instantaneous field direction are directly related to the time derivatives of the instantaneous laser electric field along …
Two-photon free-free transitions in elastic laser-assisted electron-hydrogen collisions are studied in the domain of high scattering energies and low or moderate field intensities, in the third order of perturbation theory, taking into account all the involved Feynman diagrams. Based on the analytical expressions of the transition amplitudes, the differential cross sections for two-photon absorption/emission are computed at impact energy $E_i=100$ eV. The effect of field polarizations on the angular distribution and on the frequency dependence of the differential cross section is analyzed.
Laser spectroscopy studies are being prepared to measure the 1s ground state hyperfine splitting in trapped cold highly charged ions. The purpose of such experiments is to test quantum electrodynamics in the strong electric field regime. These experiments form part of the HITRAP project at GSI. A brief review of the planned experiments is presented.
The dynamics of the electron impact multiphoton ionization of a hydrogen atom in the presence of an intense laser field has been studied theoretically, with a view to comparing (qualitatively) the results with the recent kinematically complete experiments of Horr et al [ Phys. Rev. Lett., vol. 94, 153201, (2005) ] for the He target. Significant laser modifications are noted in the present doubly (DDCS) and the fully differential cross sections (TDCS). For most of the explored kinematics (chosen in accordance with the experiment), the present binary peak intensity of the laser-assisted TDCS is …
We propose a method to study the tunneling process by analyzing the time-dependent ionization yield in circularly polarized laser. A numerical calculation shows that for an atom exposed to a long laser pulse, if its initial electronic state wave function is non-spherical symmetric, the delayed phase shift of the ionization rate vs. the laser cycle period in real time in the region close to the peak intensity of the laser pulse can be used to probe the tunneling time. In this region, an obvious delay phase shift is observed, showing the tunneling time is in order of tens of attoseconds. Further…
We investigate the polarization properties of high harmonics generated with the bichromatic counterrotating circularly polarized (BCCP) laser fields by numerically solving time-dependent Schrödinger equation (TDSE). It is found that, the helicity of the elliptically polarized harmonic emission is reversed at particular harmonic orders. Based on the time-frequency analysis and the classical three-step model, the correspondence between the positions of helicity reversions and the classical trajectories of continuum electrons is established. It is shown that, the electrons ionized at one lobe of …
High-harmonic generation from hydrogenlike muonic atoms exposed to ultraintense high-frequency laser fields is calculated. Systems of low nuclear charge number Z are considered where a nonrelativistic description applies. By comparing the radiative response for different isotopes we demonstrate characteristic signatures of the finite nuclear mass and size in the harmonic spectra. In particular, for Z>1, an effective muon charge appears in the Schrödinger equation for the relative particle motion, which influences the position of the harmonic cutoff. Cutoff energies in the MeV domain can be ach…
We present a theoretical study of the ionization dynamics of a hydrogen atom exposed to attosecond laser pulses in the extreme ultra violet region at very high intensities. The pulses are such that the electron is expected to reach relativistic velocities, thus necessitating a fully relativistic treatment. We solve the time dependent Dirac equation and compare its predictions with those of the corresponding non-relativistic Schr{ö}dinger equation. We find that as the electron is expected to reach about 20% of the speed of light, relativistic corrections introduces a finite yet small decrease i…
We investigated the coherence of spin-polarized ^{87}Sr atoms trapped in a light-shift-free one-dimensional optical lattice during their interaction with a clock laser on the ^1S_0-^3P_0 transition. Collapses and revivals appeared for more than 50 Rabi cycles, attributed to the thermal distribution of discrete vibrational states in the lattice potential. The population oscillation in the clock states lasted more than 1s, demonstrating high immunity from decoherence. This long atomic coherence suggests the feasibility of Pauli blocking of collisions in optical clock excitation.
Within the framework of the coplanar binary geometry where it is justified to use plane wave solutions for the study of the $(e,2e)$ reaction and in the presence of a circularly polarized laser field, we introduce as a first step the DVRPWBA1 (Dirac-Volkov Plane Wave Born Approximation1) where we take into account only the relativistic dressing of the incident and scattered electrons. Then, we introduce the DVRPWBA2 (Dirac-Volkov Plane Wave Born Approximation2) where we take totally into account the relativistic dressing of the incident, scattered and ejected electrons. We then compare the cor…
We theoretically study multiphoton ionization of Mg in the circularly as well as the linearly polarized laser fields. Specifically two-, three-, and four-photon ionization cross sections from the ground and first excited states are calculated as a function of photon energy. Calculations are performed using the frozen-core Hartree-Fock and also the model potential approaches and the results are compared. We find that the model potential approach provide results as good as or even slightly better than those by the frozen-core Hartree-Fock approach. We also report the relative ratios of the ioniz…
The generation of harmonics by atoms interacting with two circularly polarized and frequency related laser fields is addressed through ab initio numerical simulations. A detailed charaterization of a few specific harmonics is given. In particular, the two different cases where the total energy absorbed through photons is far-off or close to the energy gap between different atomic states are investigated. It is found that the conversion efficiency in the harmonic generation is strongly dependent on the inner atomic structure and in certain specific cases it can be significantly enhanced within …
We present accurate measurements of carrier-envelope phase effects on ionisation of the noble gases with few-cycle laser pulses. The experimental apparatus is calibrated by using atomic hydrogen data to remove any systematic offsets and thereby obtain accurate CEP data on other generally used noble gases such as Ar, Kr and Xe. Experimental results for H are well supported by exact TDSE theoretical simulations however significant differences are observed in case of noble gases.
We demonstrate sub-cycle control of frustrated double ionization (FDI) in the two-electron triatomic molecule D$_3^+$ when driven by two orthogonally polarized two-color laser fields. We employ a three-dimensional semi-classical model that fully accounts for the electron and nuclear motion in strong fields. We control FDI triggered by a strong near-infrared laser field with a weak mid-infrared laser field. This control as a function of the time delay between the two pulses is demonstrated when the FDI probability and the distribution of the momentum of the escaping electron along the mid-infra…
We examine correlated electron and doubly charged ion momentum spectra from strong field double ionization of Neon employing intense elliptically polarized laser pulses. An ellipticity-dependent asymmetry of correlated electron and ion momentum distributions has been observed. Using a 3D semiclassical model, we demonstrate that our observations reflect the sub-cycle dynamics of the recollision process. Our work reveals a general physical picture for recollision-impact double ionization with elliptical polarization, and demonstrates the possibility of ultrafast control of the recollision dynami…
We demonstrate the control of electron-electron correlation in frustrated double ionization (FDI) of the two-electron triatomic molecule D$_{3}^{+}$ when driven by two orthogonally polarized two-color laser fields. We employ a three-dimensional semi-classical model that fully accounts for the electron and nuclear motion in strong fields. We analyze the FDI probability and the distribution of the momentum of the escaping electron along the polarization direction of the longer wavelength and more intense laser field. These observables when considered in conjunction bear clear signatures of the p…
We demonstrate a novel compact solid-state laser source for high-resolution two-photon spectroscopy of the $1S-2S$ transition in atomic hydrogen. The source emits up to 20 mW at 243 nm and consists of a 972 nm diode laser, a tapered amplifier, and two doubling stages. The diode laser is actively stabilized to a high-finesse cavity. We compare the new source to the stable 486 nm dye laser used in previous experiments and record 1S-2S spectra using both systems. With the solid-state laser system we demonstrate a resolution of the hydrogen spectrometer of 6 \times 10^{11} which is promising for a…
The HERMES hydrogen and deuterium nuclear-polarized gas targets have been in use since 1996 with the polarized electron beam of HERA at DESY to study the spin structure of the nucleon. Polarized atoms from a Stern-Gerlach Atomic Beam Source are injected into a storage cell internal to the HERA electron ring. Atoms diffusing from the center of the storage cell into a side tube are analyzed to determine the atomic fraction and the atomic polarizations. The atoms have a nuclear polarization, the axis of which is defined by an external magnetic holding field. The holding field was longitudinal dur…
We study nonperturbative multiphoton electron-positron pair creation in ultrastrong electromagnetic fields formed by two counterpropagating pulses with elliptic polarization. Our numerical approach allows us to take into account the temporal as well as the spatial variation of the standing electromagnetic field. The spin and momentum resolved pair creation probabilities feature characteristic Rabi oscillations and resonance spectra. Therefore, each laser frequency features a specific momentum distribution of the created particles. We find that, depending on the relative polarization of both pu…
Increasing ellipticity usually suppresses the recollision probability drastically. In contrast, we report on a recollision channel with large return energy and a substantial probability, regardless of the ellipticity. The laser envelope plays a dominant role in the energy gained by the electron, and in the conditions under which the electron comes back to the core. We show that this recollision channel eciently triggers multiple ionization with an elliptically polarized pulse.
We describe methods for the production of spin-polarized H and D atoms from the IR-excitation and photodissociation of molecular beams of HBr, HI, and ${\rm NH_{3}}$ isotopes, including optical excitation schemes with partial hyperfine resolution. We discuss the extent to which the production rates may approach the IR-laser production rates of ${\rm 10^{21}\, photons\, s^{-1}}$, and how the production rates of conventional methods of ${\sim}{\rm 10^{17} \, s^{-1}}$ may be surpassed significantly.
We consider the formation of RbCs by an elliptically polarized laser pulse. By varying the ellipticity of the laser for sufficiently large laser intensity, we see that the formation probability presents a strong dependence, especially around ellipticity 1/ $\sqrt$ 2. We show that the analysis can be reduced to the investigation of the long-range interaction between the two atoms. The formation is mainly due to a small momentum shifts induced by the laser pulse. We analyze these results using the Silberstein's expressions of the polarizabilities, and show that the ellipticity of the field acts …
Theoretical study on H$_2^+$ in an intense infrared laser field on the attosecond time-scale reveals that the molecular ion shows multiple bursts of ionization within a half-cycle of the laser field oscillation, in contrast to the widely accepted tunnel ionization picture for an atom. These bursts are found to be induced by transient localization of the electron at one of the nuclei, and a relation between the time instants of the localization and the vector potential of the laser light is derived. Furthermore, an experimental scheme is proposed to probe the localization dynamics by an extreme…
We present a study of Mott scattering of polarized electrons in the presence of a laser field with circular polarization using the helicity formalism and the introduction of the well known concept of non flip differential cross section as well as that of flip differential cross section. The results we have obtained in the presence of a laser field are coherent with those obtained in the absence of a laser field. We have studied the relativistic regime as well as the non relativistic regime that are precisely defined in the text. Two important consistency checks have been carried out successful…
We demonstrate a \SI{650}{\mW} \SI{243}{\nm} continuous-wave laser coupled to a linear optical enhancement cavity. The enhancement cavity can maintain $>$\SI{30}{\W} of intracavity power for \SI{1}{\hour} of continuous operation without degradation. This system has sufficient power for a demonstration of two-photon laser cooling of hydrogen and may be useful for experiments on other simple two-body atomic systems.
A theoretical study of the intense-field single ionization of molecular hydrogen or deuterium oriented either parallel or perpendicular to a linear polarized laser pulse (400 nm) is performed for different internuclear separations and pulse lengths in an intensity range of $(2-13)\times10^{13} $W cm$^{-2}$. The investigation is based on a non-perturbative treatment that solves the full time-dependent Schrödinger equation of both correlated electrons within the fixed-nuclei and the dipole approximation. The results for various internuclear separations are used to obtain the ionization yields of…
We show that circularly polarized femtosecond laser pulses produce field-free alignment in linear and planar molecules. We study the rotational wavepacket evolution of O$_2$ and benzene created by circularly polarized light. For benzene, we align the molecular plane to the plane of polarization. For O$_2$, we demonstrate that circular polarization yields a net alignment along the laser propagation axis at certain phases of the evolution. Circular polarization gives us the ability to control alignment of linear molecules outside the plane of polarization, providing new capabilities for molecula…
A way to considerably enhance terahertz radiation, emitted in the interaction of intense mid-infrared laser pulses with atomic gases, in both the total energy and the electric-field amplitude is suggested. The scheme is based on the application of a two-color field consisting of a strong circularly polarized mid-infrared pulse with wavelengths of $1.6÷4\,μ{\rm m}$ and its linearly or circularly polarized second harmonic of lower intensity. By combining the strong-field approximation for the ionization of a single atom with particle-in-cell simulations of the collective dynamics of the generate…
We present theoretical calculations for polarization and ellipticity of high-order harmonics from aligned N$_2$, CO$_2$, and O$_2$ molecules generated by linearly polarized lasers. Within the rescattering model, the two polarization amplitudes of the harmonics are determined by the photo-recombination amplitudes for photons emitted parallel and perpendicular to the direction of the {\em same} returning electron wave packet. Our results show clear species-dependent polarization states, in excellent agreement with experiments. We further note that the measured polarization ellipse of the harmoni…
We study elementary particle reactions that result from the interaction of an atomic system with a very intense laser wave of circular polarization. As a specific example, we calculate the rate for the laser-driven reaction $e^+e^- \to μ^+μ^-$, where the electron and positron originate from a positronium atom or, alternatively, from a nonrelativistic $e^+e^-$ plasma. We distinguish accordingly between the coherent and incoherent channels of the process. Apart from numerical calculations, we derive by analytical means compact formulas for the corresponding reaction rates. The rate for the coher…
The nondirect product discrete variable representation (npDVR) is developed for the time-dependent Schrödinger equation with nonseparable angular variables and is applied to a hydrogen atom in elliptically polarized strong laser fields. The 2D npDVR is constructed on spherical harmonics orthogonalized on the 2D angular grids of the Popov and Lebedev 2D cubatures for the unit sphere. With this approach we have investigated the dynamics of a hydrogen atom initially in its ground state in elliptically polarized laser fields with the intensity up to $I=10^{14}$ W/{cm}$^2$ and wavelength of $λ=800$…
We calculate the lifetime of both the o-Ps and the p-Ps positronium annihilation decay $ Ps\toγγ$ in the strong circular polarized laser field. We take a strategy of the factorization to separate the effects caused by the Coulomb interaction and the strong laser field interaction. It is factorized in the time direction but not in the space direction. Our results show that in the laser with long wavelength and high intensity, the lifetimes of those Ps states are dramatically increased. For $\rm CO_2$ laser with $10\, μm$ wavelength and $10^{13} W/cm^2$ intensity, lifetime of the spin-single pos…
